Deep in British Columbia, at the turn of the 20th century, lies Winteridge: a small village, perched on a lake made famous by the monster that lives within its depths. Twelve-year-old Iris Sparks lives in Winteridge with her brother, her working-class Welsh mother, Llewellyna, and her blue-blooded father from England, a progressive bohemian who has brought his family to Canada for an adventure. But amid the idyllic, Edwardian setting, there are dangers lurking, and Iris watches in horror as her mother increasingly succumbs to seizures. Llewellyna has visions of disasters yet to come, and Iris's mind wills these visions to life, blending Welsh myths with Indigenous stories about what haunts the lake. As Iris grows older, she falls into a toxic love triangle, and all the while, the lake monster - the addanc - watches and waits. As the First World War reaches its height, and Iris's mind continues to exert its strange and awesome power, she and her community in Winteridge must find a way to survive while living at the mercy of unknown beasts and a hungry darkness.